1. Field of the Invention
The present invention relates to polysulfide measurement methods, more particularly to polysulfide measurement when polysulfides are produced or consumed in processes. Polysulfides are produced in white liquor used in paper production, and consumed in other processes.
2. Related Art
Polysulfide is a sulfur compound used in various industries for various purposes. For example, in the pulp and paper industry, it is a very well established fact that the use of polysulfide during the cooking process increases the pulp yield (based on wood). It is beneficial to the pulp industry since they can produce more pulp from a given ton of wood (or) reduce the usage of wood for given ton of pulp. One method of producing polysulfide is to oxidize white liquor, which contains sodium sulfide, with oxygen. Since polysulfide is an intermediate compound, the reaction must be well controlled; or else, the oxidized white liquor will contain unacceptable quantities of thiosulfate, sulfite and sulfate. Until today the challenge is having a simple method to measure the concentration of polysulfide especially on an industrial scale. All current available methods to measure the concentration of polysulfide are analytical methods and are therefore difficult to integrate to an industrial process.
Polysulfide can be generated by different means based on industry and type of use. In the pulp and paper industry, adding elemental sulfur to the white liquor can form polysulfides. See for example, Casey, J. P., “Pulp and Paper Chemistry and Chemical Technology”, Third Edition, Volume I, Wiley-Interscience Publication, p 432. However, adding elemental sulfur to the white liquor leads to imbalances in the sulfur balance of the chemical recovery cycle. The build-up of sulfur will eventually be eliminated to the atmosphere as sulfur gas emission. The second approach consists of chemically oxidizing the sodium sulfide present in the white liquor to sodium polysulfide using MnO2 as a catalyst. The resulting liquor is known in the art as orange liquor. The methods involve several chemical species. One goal sought during the oxidizing process is to selectively generate polysulfide and minimize the formation of dead load, more specifically thiosulfate.
Several variations of the oxidative methods have been published. U.S. Pat. No. 3,470,061, Barker discloses using an inorganic manganese oxide as the oxidant. In U.S. Pat. No. 4,024,229, Smith discloses a method to generate polysulfide by chemical oxidation using particulate carbon, coated with PTFE, as the catalyst. The method is said to reduce production of thiosulfate. In U.S. Pat. No. 5,082,526, Dorris discloses a method to produce polysulfide in the presence of lime mud. In U.S. Pat. No. 5,624,545 Landfors et al, discloses a method to produce polysulfide by electrolysis of the white liquor.
Polysulfide can also be generated electrochemically. See, for example, Watanabe, K., et al., “New Process of Producing Highly Concentrated Polysulfide Liquor by Electrolysis of White Liquor”, TAPPI 1999 Pulping Conference Proceedings, Volume 1.
Polysulfide is used for various other purposes in addition to its use in increasing pulp yield in pulp and paper mills. One other use of polysulfide is in the preparation of sealants. Polysulfide is considered to be thermoset sealant. Evode Ltd. discusses an example of a monopoly polysulfide sealant in product data sheet, and markets the sealant under the trade designation “MONOPOL”. The data sheet reports that sealants prepared using monopol polysulfide provide more movement accommodation than competitive polysulfides, and forms a tough compound with good adhesion and color retention on reaction with atmospheric moisture.
U.S. Pat. No. 5,075,098 discusses the preparation of sodium monosulfide by means of reacting a sodium polysulfide with sodium under protective gas. U.S. Pat. No. 5,215,865 discusses an image development method and the preferred developer is an aqueous solution of sodium sulfide and ammonium polysulfide. U.S. Pat. No. 6,279,733 discusses an invention related to a tire having a sidewall component comprising an EPDM-based rubber composition prepared with specified precipitated silica reinforcement and an organosilane disulfide material. A liquid organosilane polysulfide comprising bis-(3-ethoxysilylpropyl) polysulfide is discussed.
Polysulfide can be associated with alkaline, alkaline earth and transition metals. U.S. Pat. No. 3,890,428 discusses the removal of coloring agents such as polysulfide in the manufacture of ammonium thiosulfate solutions using aqueous sodium, lithium or potassium silicates. Nathalie et al., J. Appl. Phys., Part 1, Rgul Pap, short Note, vol. 31, n 9A, September 1992, p 2786-2790, discuss the use of a potassium polysulfide flux to prepare efficient phosphors. It was also shown by Vasilyeva et al., Journal of Alloys and Compounds, vol. 268, n 1-2, Mar. 27, 1998, p 72-77, that heterogeneity such as sodium or cerium polysulfides in small amounts can be the cause of a significant modification of color in gamma-[Na]—Ce2S3 solid solutions.
Polysulfide elastomers are known, which are synthetic polymers in either solid or liquid form obtained by the reaction of sodium polysulfide with organic dichlorides such as dichlorodiethyl formal, alone or mixed with ethylene dichloride. Polysulfide elastomers are said to be outstanding for resistance to oils and solvents and for impermeability to gases. Hawley's Condensed Chemical Dictionary, Twelfth Ed., page 941 (1993).
Since human vision reduces many wavelength bands in a light spectrum into a three-dimensional signal in the retina, color is conventionally expressed as calorimetric quantities having three values. A common colorimetric system is the CIE L*a*b*. The term CIE corresponds to the International Commission on Illumination (abbreviated CIE from the French expression). In this color system +a* corresponds to red, −a* to green, +b* to yellow, and −b* to blue. The L* values correspond to the lightness scale. Customarily, a numerical expression of such a color difference is used to determine acceptability of manufactured items. With the CIE L*a*b* colorimetric system, numerical expressions to express color differences exist.
Methods exist for the estimation of composition and other properties from all types of spectral measurements, (e.g., reflectance and transmittance spectral measurements) by reference to sets of calibration data measured on samples (including liquids) of known properties. For example, U.S. Pat. No. 4,800,279, discloses a method using infrared absorbance spectra of calibration samples of known physical properties to determine those infrared wavelengths at which the absorbance correlates with a physical property to be quantified, and then estimate that property for a sample from its infrared absorbance spectrum. U.S. Pat. No. 5,121,337 and U.S. Pat. No. 5,446,681 discloses methods for estimating unmeasured properties, such as composition, from spectral measurements on samples, using advanced statistical procedures and rule-based criteria to correlate spectral measurements and measurements of the desired property (or composition) for a set of calibration samples. In the case of patent U.S. Pat. No. 5,121,337 the emphasis is placed on correcting spectral data due to the measurement process itself; whereas U.S. Pat. No. 5,446,681 is geared towards the on-line spectrometry aspect.
The main difference between the previously mentioned patents, and our invention, is that they do not operate in the visible light spectrum. They all use infrared. Another important distinction, and this corresponds to U.S. Pat. No. 5,121,337 and U.S. Pat. No. 5,446,681, is that they require complex statistical methods for data analyses. The methods described in this invention disclosure, to detect variations in polysulfide concentration, is based on color difference indicators.
Another patent of interest is U.S. Pat. No. 5,616,214. In this patent, a direct monitoring and control method is provided for on-line measurement of effective alkali, carbonate, sulfate and thiosulfate concentrations in process liquors for the production of kraft pulp. However, unlike our invention, the device/strategy does not monitor polysulfide concentration, and, as in the previous examples, the apparatus operates at infrared.
The inventors believe that the relation between the L*a*b* values and polysulfide concentration can be affected by the chemical and physical properties of the composition being tested (for example, hydroxyl ion concentration, temperature, impurities, and the like), as noted in the discussion of Related Art. For example, it is known that white liquor is a solution of Sodium Hydroxide and Sodium Sulfide (which are the two main pulping components). The concentration of these compounds can vary depending on the mill and the type of product desired. Many industrial liquors contain compounds that are useless and commonly referred as dead load. Examples of dead load are Na2CO3, Na2S2O3, Na2SO3, and Na2SO4. These industrial liquors also contain impurities in small amount such as Magnesium, Potassium, Phosphorus, Silicon, Iron, Aluminum, Barium. Therefore a calibration is specific to a certain situation, mill, or process. This observation is based on the experience of the inventors and literature. For example, Ants Teder, “Spectrophotometric Determination of Polysulfide Excess Sulfur in Aqueous Solutions”, Svensk Papperstidning, No. 6, 31 mars 1967, has documented the possible effect of hydroxyl ions concentration: “Only a few spectrophotometric studies of aqueous polysulfide solutions have been reported. They are of rather small scope, being minor parts of investigations using mainly other methods, and are limited to wavelengths longer than 295 nm. Great difficulties in reproducing the spectra have been reported. These problems can be overcome by handling the samples under careful exclusion of air, control of hydroxyl ions concentration and by avoiding too small polysulfide concentrations.” Also, it is well known that, when the mixture of molten sodium sulfide and sodium carbonate produced by a recovery furnace is dissolved in water, a green solution, called “green liquor”, is obtained. This green color has been ascribed to iron impurities. Green R. P., et al., “Chemical Recovery in the Alkaline Pulping Process”, Third Edition, TAPPI PRESS, Atlanta, Ga., p. 3. The spectra of solutions (containing sulfide and polysulfide) are affected by temperature. Teder, Ants, “The Spectra of Green Sulfide and Polysulfide Solutions”, Svensk Papperstidning, No. 11, 15 Juni 1968.
The currently available methods to measure polysulfide concentrations are all analytical. The methods are time consuming and, depending upon the method, expensive analytical tools are necessary. These laboratory oriented methods cannot be implemented to measure and control large industrial processes.